The advantages of frying food products in a reduced pressure atmosphere or vacuum are well understood. Where the cooking environment is in a range of about a minus 14.0 psi absolute or 28.5 mm Hg vacuum, foods can be cooked at lower temperatures. In the case where cooking oil or fats are the cooking medium effective temperatures may be on the order of 120° C. to about 140° C. In this temperature range the development of acrylamides in food products is significantly reduced, as is oil uptake by the product and the product quality is enhanced. The Maillard reaction, a form of nonenzymatic browning similar to carmelization, is also better controlled within the lower pressure, lower temperature parameters. As is known, the Maillard reaction results from a chemical reaction between an amino acid and a reducing sugar usually in the presence of heat. At high temperatures, acrylamide can be formed which is undesirable.
Workers in this field have recognized these benefits and have developed equipment to vacuum fry a variety of products including potato chips/crisps and the like. The prior art vacuum cooking equipment generally followed either one of two design configurations. First, a batch cooker was used in a cycle where product to be cooked was placed in a chamber sufficiently strong to sustain a vacuum, the chamber was then evacuated, cooking heat applied for the desired period after which the chamber was reopened after breaking the vacuum. Product was then removed and the cycle repeated as is common in batch cooking procedures. Although the resulting cooked food product was satisfactory, production output was low and equipment costs high in view of the low production volume. The chamber cleaning and maintenance was cumbersome and time consuming. This configuration is typified by the Tippmanm U.S. Pat. No. 5,767,487 granted Jun. 16, 1998.
Second, a removable fryer apparatus operatively arranged within a pressure vessel was a design configuration adapted to achieve continuous production vacuum frying. This is typified by the Hashiguchi, at al U.S. Pat. No. 5,988,051 granted Nov. 23, 1999; the Van Der Doe U.S. Pat. No. 6,929,812 granted Aug. 16, 2005; and the Yang U.S. Pat. No. 4,852.475 granted Aug. 1, 1989. The removable fryer-in-a-pressure-vessel configuration was costly to manufacture and occupied a large floor space in the plant given its production capacity. This was due to the need for a maintenance zone and an external fryer support structure outside of the pressure vessel into which the fryer could be shifted for servicing. Additionally, the pressure vessel, inside of which the fryer operated, required a large sealable hatch at one end so that the fryer could be moved in and out for cleaning and maintenance. The manufacturing costs and sizable plant operating space were recognized disadvantages for the fryer-in-a-pressure-vessel configuration.